1. Field of the Invention
The present invention relates to a semiconductor device typified by a thin film transistor and to a fabrication method thereof.
2. Description of Related Art
Hitherto, a thin film transistor using a silicon film, has been known as well as technology for forming the thin film transistor by using silicon film formed on a glass substrate or quartz substrate.
The glass substrate or quartz substrate is used because the thin film transistor is used for an active matrix type liquid crystal display. While a thin film transistor has been formed by using an amorphous silicon film in the past, attempts have been made to fabricate the thin film transistor by utilizing a silicon film having a crystallinity (referred to as xe2x80x9ccrystalline silicon filmxe2x80x9d hereinbelow) in order to enhance its performance.
The thin film transistor using crystalline silicon film allows operation at a high speed as compared to one using amorphous silicon film. Therefore, while peripheral driving circuits of an active matrix liquid crystal display have been composed of external IC circuits, they may be built on the glass substrate or quartz substrate similarly to the active matrix circuit.
Such structure is very advantageous in miniaturizing the whole apparatus and in simplifying the fabrication process, thus leading to the reduction of the fabrication cost.
In general, a crystalline silicon film has been obtained by forming an amorphous silicon film by means of plasma CVD or low pressure thermal CVD and then by crystallizing it by performing a heat treatment or by irradiating laser light.
However, it has been the fact that it is difficult to obtain a required crystallinity across the wide area through the heat treatment because it may cause nonuniformity in the crystallization.
Further, although it is possible to obtain the high crystallinity partly by irradiating laser light, it is difficult to obtain a good annealing effect across the wide area. In particular, the irradiation of the laser light is apt to become unstable under the condition for obtaining the good crystallinity.
Meanwhile, a technology described in Japanese Patent Laid-Open No. Hei. 6-232059 has been known. This technology provides a crystalline silicon film through a heat treatment at a lower temperature than that of the prior art by introducing a metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film.
This technology allows high crystallinity to be obtained uniformly across a wide area as compared to the prior art crystallization method by way of bulk heating or crystallization of an amorphous silicon film selectively by means of irradiation of laser light.
However, it is difficult to obtain a crystalline silicon film having high crystallinity and uniformity across a wide area which is required for an active matrix type liquid crystal display.
Further, because the metal element is contained within the film and an amount thereof to be introduced has to be controlled very carefully, there is a problem in its reproducibility and stability (electrical stability of a device obtained).
Still more, there is a problem that an elapsed change of the characteristics of a semiconductor device to be obtained is large or an OFF condition leakage value, in the case of a thin film transistor, is large, for example due to the influence of the remaining metal element. That is, although the metal element which promotes the crystallization of silicon plays the useful role in obtaining the crystalline silicon film, its existence becomes a negative factor which causes various problems after obtaining the crystalline silicon film.
It is an object of the invention disclosed in the present specification to provide a semiconductor device having excellent characteristics by using a crystalline silicon film having a high crystallinity.
It is an object of the invention disclosed in the present specification to provide a technology for reducing concentration of a metal element within a crystalline silicon film obtained by utilizing the metal element which promotes crystallization of silicon.
It is another object of the present invention to provide a technology which can enhance characteristics and reliability of the semiconductor device thus obtained.
One of the inventions disclosed in the present specification comprises steps of forming an amorphous silicon film on a substrate having an insulated surface; intentionally introducing a metal element which promotes crystallization of silicon to the amorphous silicon film; obtaining a crystalline silicon film by crystallizing the amorphous silicon film by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by patterning the crystalline silicon film; removing or reducing the metal element existing within the crystalline silicon film by performing a second heat treatment within an oxidizing atmosphere containing a halogen element; removing a thermal oxide film formed in the previous step; and forming another thermal oxide film after removing the thermal oxide film by performing another thermal oxidation, and that a temperature of the second heat treatment is higher than that of the first heat treatment.
An arrangement of another invention comprises steps of forming an amorphous silicon film on a substrate having an insulated surface; intentionally introducing a metal element which promotes crystallization of silicon, to the amorphous silicon film; obtaining a crystalline silicon film by crystallizing the amorphous silicon film by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by patterning the crystalline silicon film; performing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the crystalline silicon film to be gettered to a thermal oxide film to be formed; removing the thermal oxide film formed in the previous step; and forming another thermal oxide film after removing the thermal oxide film by performing another thermal oxidation, and that a temperature of the second heat treatment is higher than that of the first heat treatment.
An arrangement of another invention comprises steps of forming an amorphous silicon film on a substrate having an insulated surface; intentionally and selectively introducing a metal element which promotes crystallization of silicon to the amorphous silicon film; growing crystal in a direction parallel to the film from a region of the amorphous silicon film into which the metal element has been intentionally and selectively introduced by a first heat treatment in the temperature range of 750xc2x0 C. to 1100xc2x0 C.; forming an active layer of the semiconductor device by using a region in which crystal has been grown in the direction parallel to the film by patterning; performing a second heat treatment within an oxidizing atmosphere containing a halogen element to cause the metal element existing within the active layer to be gettered to a thermal oxide film to be formed; removing the thermal oxide film formed in the previous step; and forming another thermal oxide film after removing the thermal oxide film by performing another thermal oxidation, and that a temperature of the second heat treatment is higher than that of the first heat treatment.
In the above-mentioned arrangements, it is preferable to use a quartz substrate as the substrate from the aspect of heat resistance.
Further, when a thin film transistor is to be fabricated, it is preferable to form a gate insulating film by utilizing the another thermal oxide film.
Still more, one or a plurality elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au may be used as the metal element which promotes the crystallization of silicon.
The concentration of oxygen contained in the amorphous silicon film, i.e. the starting film, is preferably 5xc3x971017 cmxe2x88x923 to 2xc3x971019 cmxe2x88x923. This is an important point in obtaining the effect for gettering the metal element which promotes the crystallization of silicon.
The concentration of impurity and element in the present specification is defined as the minimum value measured by SIMS (secondary ion mass spectrometry).
According to a preferred mode for carrying out the invention disclosed in the present specification, an amorphous silicon film is formed on the quartz substrate at first. Then, the amorphous silicon film is crystallized by an action of metal element typified by nickel which promotes crystallization of silicon to obtain a crystalline silicon film.
This crystallization is performed in temperature of 750xc2x0 C. to 1100xc2x0 C. to maximize the action and to obtain the crystalline silicon film having high crystallinity.
The metal element is contained in the crystalline silicon film in the state in which the film has been crystallized by the above-mentioned heat treatment.
Next, the active layer of the semiconductor device, e.g. an active layer of a thin film transistor, is formed by patterning.
Another heat treatment is performed within an oxidizing atmosphere containing a halogen element (e.g. HCl) to form a thermal oxide film on the surface of the pattern of the active layer made of the crystalline silicon film.
At this time, the gettering effect may be not fully obtained if the temperature of the heat treatment is too high because the film forming speed of the thermal oxide film might become too fast. In such a case, the film forming speed of the thermal oxide film may be controlled by diluting oxygen by inert gas.
Then, the metal element is gettered into the thermal oxide film to be formed by the action of the halogen element and the concentration of the metal element within the crystalline silicon film is reduced.
The heat treatment for gettering the nickel element is preferable to perform at a temperature higher than that of the heat treatment for the crystallization.
As a result of the heat treatment for gettering, a thermal oxide film containing the nickel element in high concentration is formed. Then, the crystalline silicon film having the high crystallinity and having low concentration of the metal element may be obtained by removing this thermal oxide film.